UIViews by default only handle a single touch event at once. If you
want your view to handle multiple touches, you must set the
UIView.MultipleTouchEnabled to true.

iOS creates a T:UIKit.UIEvent object any time a finger
touches the screen, moves or is removed from the screen. The UIEvent encapsulates all of the touches
that are taking place on the screen at this point, even those that
do not belong to this view. In addition to the UIEvent, there is an NSSet containing UITouch objects that represent the state
of each finger on the screen for this particular view.